Custodian of Oxygen Monitoring: Is There a Winner?

How to cite this article: Praveen Kumar G, Kakar V. Custodian of Oxygen Monitoring: Is There a Winner? Indian J Crit Care Med 2021;25(9):967-968.

Hypoxemia is extremely common in critically ill patients. In a multicenter study, over 50% of the patients evaluated had some degree of hypoxemia and close to 27% of the patients with hypoxemia died in the hospital.[1] This makes a point for close and continuous monitoring of patients with hypoxemia. Measuring peripheral oxygen saturation (SpO2) by pulse oximetry and dissolved oxygen in the arterial blood (SaO2) remains the most validated and common method for evaluating the degree of hypoxemia. Both the modalities have been used interchangeably for monitoring oxygen saturation, but PaO2 measurements have been more widely accepted as a method to quantify the degree of hypoxemia and to titrate inspired oxygen levels.

In a retrospective analysis, involving 300 patients, Sheetal Babu et al. showed that SF ratio can be used as an alternative to PF ratio in critically ill patients with hypoxic respiratory failure. Notably, a significant number of patients were on vasopressor and inotropic support when the measurements were made, highlighting the functionality of SpO2 in patients with good peripheral perfusion. Rice and his colleagues described the relationship between the P/F and S/F ratios with a simple equation and showed that SF and PF ratios can be interchanged across varying degrees of hypoxemia with near accuracy.[2] Likewise, multiple researchers have tried to answer the same question, if SpO2 can replace PaO2 in critical care settings, and the answer is an overwhelming YES.[3-5]

Another part of the study was to establish cutoffs of SF ratio for various PF ratios. Even if the cutoffs were established, they had a lower sensitivity and specificity. SF ratio of 285 correlated with PF of 200, and SF ratio of 323 correlated with PF ratio of 300. The cutoffs though different when compared to other studies, they were definitely not disparate. The reason for varying values in different studies could be explained by the fact that SpO2 remains the same for a wide range of PaO2.

In the study published in this edition of IJCCM, Sheetal Babu and his colleagues also tried to answer another pertinent question. If PF ratio can be replaced by SF ratio or SpO2 unambiguously with different methods of oxygen supplementation. The answer again is an overwhelming YES. The answer remained YES for both invasive and noninvasive methods of oxygen supplementation.

In the middle of the raging pandemic with thousands of patients on some form of oxygen supplementation, this study asks critical care physicians a cardinal question, and it questions the utility of arterial blood gases in measuring oxygenation and quantifying hypoxemia. The study and already existing literature are a testament to the fact that SpO2 is a reliable indicator for tissue oxygenation. Restoring the utility and benefits of SpO2 has limitless advantages. First, PaO2 is a finer indicator of oxygen content in the blood, but SpO2 also reflects upon tissue perfusion and oxygen delivery. Second, SpO2 gives a continuous measure of tissue oxygen levels and thereby precludes the delays in decision-making based on PaO2. Thirdly, in comparison to SpO2, the use of arterial blood gases is invasive, expensive, and of limited utility in measuring oxygenation. Also, PaO2-based interventions preludes to additional blood gas testing, thereby squandering resources. Fourth, albeit not yet validated, to quantify the degrees of hypoxemia and acute respiratory distress syndrome, SF ratio can be reliably used for therapeutic targets and clinical decision-making in intensive care settings. A worsening SF ratio can be reliably interchanged with the PF ratio for escalation of care in the pyramid for the treatment of hypoxic patients. Fifth, SpO2 and SF ratio can be more appropriate in the middle of the pandemic when scores of patients need repeated assessments of oxygenation and the resources are scarce and limited.

Since SpO2 remains more than 90 for a very wide range of PaO2, accepting a lower PaO2 or late diagnosis of worsening hypoxemia is a concern while using SF ratios, and the concern is not without a merit. Thus, though SF ratio can be used as a surrogate for PF ratio in wide settings, when in doubt PaO2 measurements using arterial blood gases should be considered. Also, multiple other drawbacks of pulse oximetry should be worth remembering.

So, to answer the question in the title: Is there a custodian of oxygen monitoring? The answer can definitely not be a plain sailing. We would rather reframe the question and ask which modality among the two is more beneficial? And the answer is clearer and it is definitely SpO2. Through the quotidian traffic of monitoring equipment available for intensive care physicians, SpO2 remains the simplest way of measuring hypoxemia and still remains the only continuous monitoring device ubiquitously present. We would conclude by saying that taking the road not taken might be challenging and rewarding, but one should not forget that the road not taken is not taken for a reason, and knowing the reason before can prevent adversities. Measuring SF ratio is the road not taken, and the critical care physicians should know the reasons before driving down the road.